1. Field of the Invention
This invention relates to an improvement in polymeric colorants. More particularly it involves polymeric colorants with improved solubility at acidic pH's and a method for their achievement.
2. The Prior Art
U.S. Pat. No. 3,920,855 of Dawson, et al., issued Nov. 18, 1975 and U.S. Pat. No. 4,018,826, issued Apr. 17, 1977 to Gless, et al., disclose that polymeric colorants can offer attractive properties such as inability to be absorbed through the walls of the gastrointestinal tract. This nonabsorption means much reduced and possibly eliminated risks of systemic toxicity and suggests advantageous application of stable polymeric colorants in edibles such as foods, beverages and the like. It is a common characteristic of such applications to have aqueous substrates or to at least contain an aqueous phase, in which the colorant is to be dissolved. These applications also often present acidic environments of use as the majority of food and beverage applications are acidic. Thus, an important property of a polymeric colorant is often its ability to dissolve or remain in solution in an acidic aqueous enviroment.
One way to achieve this desired acid solubility is to incorporate in the polymeric colorant anionic groups such as carboxyl groups, sufonate groups or phosphonate groups.
Now, a polymeric colorant of the type disclosed by Dawson, et al., and Gless, et al., is of two parts--a nonchromophoric often alkyl backbone and a plurality of optical chromophores covalently affixed thereto. These anionic groups can be present either attached to the backbone or as part of the chromophores. In either position they serve to impart water solubility to the final polymeric colorant product.
In the Dawson, et al., and the Gless, et al., disclosures, the chromophore units are covalently bound to the backbone through sulfonamide or amine linkages. In situations where there is very complete substitution of these amine groups by sulfonate groups or by sulfonate-containing chromophores, the character of the overall polymeric colorant is anionic and solubility in acidic aqueous environments is fully adequate. In situations where there is a substantial proportion of unsubstituted primary alkyl amines or dialkyl amines present in the polymeric colorant as can occur either with low degrees of amine substitution by aromatic chromophore units or with substitution by chromophores through an alkyl link, it is seen that the polymeric colorants often haze and precipitate from solution at acidic pH's. In light of the success of the present invention it now appears that at basic to neutral pH's such colorants have a distinctly anionic character imparted by their COO.sup.-, SO.sub.3.sup.- or PO.sub.3.sup.= groups and are soluble. It now appears that at acidic pH's the amines begin to become protonated and the polymer approaches an isoelectric state. In view of the present teachings one may explain this precipitation as that at their isoelectric point the polymeric colorants form insoluble intramolecular and cross-linked salts. This isoelectric condition generally occurs at pH's of 2--4 for amine-containing polymeric colorants. This is an unfortunate pH to have precipitation occur as it is the pH of many soft drinks and most fruit-flavored foods--major applications for food colors.
The present invention has eliminated this troublesome precipitation of primary and secondary amine-rich polymeric colorants.